Method of storing volatile substances, container for storing said substances, and flow-control method for surface flow of superfluid helium

ABSTRACT

A method of storing volatile substances comprises the steps of cooling the substances, and maintaining the saturated vapor pressure of the substances no higher than normal pressure. A container for storing liquid helium blocks superfluid surface flow of liquid helium by providing means on an inner wall or outer wall of the container. A method of controlling a surface flow of superfluid helium includes providing a flow suppressing surface within the container, and causes the surface flow of superfluid helium to impact upon the surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of storing volatilesubstances. More particularly this invention, when substances that aregaseous at room temperature are stored in a liquid state or a solidstate, is concerned with a method of storing volatile substances where aminimum loss of the substances by vaporization can be achieved.

The present invention also relates to a container suited for storingliquid helium.

The present invention further relates to a method of controllingsuperfluid surface flow of a superfluid liquid upon utilizing thesuperfluid liquid.

2. Related Background Art

Hitherto, volatile substances have been stored by puttIng them in closedtop containers, stored by putting them in high pressure bombs, or storedby cooling them.

In general, many methods of storing volatile substances that are solidor liquid at room temperature have been known.

However, there is a limit in methods of storing in a liquid or solidstate the substances that are gaseous at room temperature.

For example, substances such as liquefied natural gas, liquid air,liquid nitrogen, liquid oxygen, liquid hydrogen and liquid helium have ahigh saturated vapor pressure at room temperature, so that they areusually stored under pressure or cooling.

Accordingly, to keep these substances in a stable state for a longperiod of time, great efforts have been made on how the temperature iskept low and also how heat is prevented from coming in.

For storing these substances, commonly used are Dewar vessels or otherthermal insulating containers.

Of these substances, greater efforts are made for storing liquid helium.

This is because liquid helium has properties different from generalliquefied gases.

More specifically, liquid helium has a very low boiling point under 1atmospheric pressure and helium-4(⁴ He) liquefies at about 4.2°K andhelium-3(³ He) liquefies at about 3.2°K. For this reason it is necessaryfor storing the liquid helium to use a container made of a thermalinsulating material such as metal (e.g., stainless steel) or glasshaving a low thermal conductivity, and used are containers whose wallsare further doubled and inside of the double walls are stored vacuum,which are the so-called Dewar vessels.

Usually, in instances where the Dewar vessels are used, methods aretaken such that the inside thereof is made to have a double structure,liquid helium is poured in an inner Dewar vessel, and liquid nitrogen orthe like is filled between the inner Dewar vessel and an outer Dewarvessel, thus suppressing the inflow of heat by thermal radiation andconduction.

In the conventional methods as described above, however, it has beenunavoidable to suffer a loss due to unnecessary vaporization when thesubstances that are gaseous under a normal pressure are liquefied orsolidified for storing, particularly when they are stored for a longperiod of time.

Particularly in the case of liquid helium, and taking into account theexpense involved, loss due to vaporization is a problem that should beovercome.

Also, when liquid helium is being cooled, it transforms into a peculiarstate that it is in a superfluid state below a certain temperature.Helium-4(⁴ He) transforms into superfluid state at 2.17°K or less, andhelium-3(³ He) transforms thereinto at a far lower temperature. Onceliquid helium turns to a superfluid state, it loses its viscosity andendlessly extends in the form of a superfluid surface flow along thesurface of the container or other structures coming into contact withthe liquid helium. This surface flow can not be stopped if, for example,a mouth of the container is simply stoppered or so, and overflows fromeven a small gap between the stopper and the container without anyresistance at all. To stop such a surface flow, the container isrequired to be hermetically closed in an atomic scale, and the stoppingof the surface flow is very difficult if a reusable mechanical sealingmeans is applied, resulting in an unavoidable loss of liquid helium.

It has been hitherto considered impossible to prevent or control such asuperfluid surface flow of liquid helium. For this reason, it only hasbeen practiced within a very limited scope to carry out cooling byutilizing a high thermal conductivity in superfluidity, keeping theliquid helium stable as being cooled to about 1°K, or efficientlytransporting liquid helium by utilizing superfluidity.

It has been deemed impossible in theory to limit the surface flow to aparticular region in a stable state on the surface of an article cominginto contact with superfluid helium, or to control the magnitude orvelocity of the surface flow, and thus there has been no choice but toallow it to flow.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a methodof storing volatile substances, that can suppress the evaporation of thevolatile substances by vaporization to a minimum, and can stably storethe volatile substances for a long period of time.

A further object of the present invention is to provide a containersuited for storing liquid helium, in particular, among the volatilesubstances.

Another object of the present invention is to provide a method ofcontrolling the magnitude or flow velocity of a superfluid surface flowof liquid helium, or of limiting the region on which the surface flow iscaused.

The present invention provides a method of storing a volatile substance,comprising cooling said volatile substance, and maintaining thesaturated vapor pressure of said substance to not higher than normalpressure.

The present invention also provides a container for storing liquidhelium, comprising means for blocking a superfluid surface flow ofliquid helium, said means being provided on an inner wall or outer wallof said container.

The present invention still also provides a method of controlling asurface flow of superfluid helium, comprising controlling the surfaceflow by bringing at least a part of the surface of a structure into astate in which the surface flow passage is suppressed, the surface flowof superfluid helium being caused on said structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating the container according to the presentinvention, whose inner wall is coated with a surface flow blockingmaterial;

FIGS. 2A and 2B are views illustrating containers according to thepresent invention, whose inner wall are provided with uneveness;

FIGS. 3A and 3B are views illustrating cross sections of uneven portionsillustrated in FIGS. 2A and 2B;

FIG. 4 is a view showing a principle by which the surface flow ofsuperfluid helium is controlled;

FIGS. 5A and 5B are views illustrating an example of transportingsuperfluid helium;

FIG. 6 is a view illustrating an embodiment of the container for storingliquid helium according to the present invention;

FIGS. 7A and 7B are views illustrating examples in which methods ofcontrolling the flow of superfluid helium are employed; and

FIG. 8 is a view illustrating an applied example in which the method ofcontrolling the flow of superfluid helium is utilized.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Storing volatile substance (particularly liquefied gas) at its boilingpoint results in the presence of a gas of 1 atmospheric pressure (i.e.760 Torr) on its liquid surface to cause convection of gas, so that theinflow of heat resulting therefrom becomes unavoidable and thevaporization of the liquefied gas proceeds.

For example, liquid nitrogen boils at 77°K, and has a saturated vaporpressure of 1 atmospheric pressure (i.e., 760 Torr) at that temperature.

Accordingly, storing liquid nitrogen at about 77°K brings about anabrupt decrease in its quantity owing to vaporization.

Nitrogen has a triple point of about 63°K. When being cooled to thistemperature, liquid nitrogen is brought to have a saturated vaporpressure of about 94 Torr. Lowering further its temperature results insolidification of the liquid nitrogen, so that the saturated vaporpressure can be made lower and thus the vaporization of liquid nitrogencan be suppressed.

Also, liquid oxygen is brought to have a saturated vapor pressure of 100Torr or less at about 75°K. and further lowering its temperature to Itstriple point (about 54°K) enables the lowering of its saturated vaporpressure to 0.76 Torr.

With respect to the liquid oxygen in this state, its evaporation due tovaporization is brought lower to a great extent, as compared with liquidnitrogen in other states. The pressure can also be sufficiently reduced,and thus the liquid oxygen is brought to undergo less convection and canbe stably stored for a long period of time.

Liquid hydrogen is brought to have a saturated vapor pressure of 100Torr or less at about 15°K, and further lowering its temperature to itstriple point (about 13.8°K) enables the lowering of its saturated vaporpressure to 52 Torr.

These liquefied gases are solidified with drop of temperature and theresulting solids require a relatively large heat for vaporization, andalso cause no troublesome phenomenon such as superfluidity, so that theycan be readily stored under reduced pressure.

As above, the loss of volatile substances due to unnecessaryvaporization can be suppressed to a minimum by cooling them and alsomaintaining the saturated vapor pressure thereof to not higher thannormal pressure.

In the present invention, it is effective to set the saturated vaporpressure of the volatile substances at 300 Torr or less, preferably 100Torr or less, and more preferably 50 Torr or less.

On the other hand, in the case of liquid helium, the method describedabove requires further elaborated measures taking into account of thefact that liquid helium does not solidify until the temperature comes toabsolute zero unless under pressure and that it is brought into asuperfluid state at 2.17°K or less, showing remarkably differentproperties from other liquefied gases.

In general, it is necessary for stably storing liquid helium to;

(1) suppress the inflow of heat due to the convection of helium vapor;

(2) suppress the surface flow of superfluidity;

(3) suppress the inflow of heat due to thermal conduction; and

(4) suppress the inflow of radiation heat.

In regard to (3) and (4) of the above, various methods have beenhitherto employed, one of which is the above double-structured Dewarvessel.

In regard to (1), no measure has been hitherto taken when helium isstored. This is because subjecting liquid helium to reduced pressure hasbeen usually avoided as the evaporation of liquid helium progresses.

It is true that the above reasoning is correct when liquid helium isstored at its temperature of 4.2°K, but the reasoning becomes incorrectif the temperature of liquid helium is further lowered to reach 2.5°K orless.

Under conditions of such a low temperature, the saturated vapor pressureof liquid helium comes to about 100°Torr, and the inflow of heat owingto the convection of helium vapor Is suppressed within this temperaturerange.

In particular, if liquid helium temperature reaches about 1° to 1.5°K,its saturated vapor pressure comes to be about 0.1°to 1°Torr and thevapor pressure on the liquid surface does not rise any longer, so thatthe convection of helium vapor becomes almost negligible also resultingin a great decrease in the evaporation rate.

The above effect becomes particularly remarkable when the saturatedvapor pressure of helium is set at 0.1 Torr or less.

However, ⁴ He is brought into a superfluid state at 2.17°K or less.Accordingly, the liquid helium creeps up on the wall of a container tocome out of the container. At this time the surface area of the liquidhelium greatly increases, resulting in promotion of the evaporation ofliquid helium and increasing the amount of lossed helium.

It has been also impossible to carry out cooling under reduced pressurewith respect to the liquid helium having been brought into a superfluidstate. This is because enlargement of its liquid surface owing to itssuperfluidity properties makes it necessarY to carry out cooling of theliquid helium to 1°K or less, and it has been impossible to carry outcooling thereof to such a temperature.

Now, in the present invention, means for blocking the superfluid surfaceflow is provided on the inner wall or outer wall of a container forstoring liquid helium, blocking the superfluidity of liquid helium.

Available as the above means is to treat the inner wall or outer wall ofthe container with use of a material for blocking the superfluid surfaceflow, or to provide uneveness thereon for blocking the superfluidsurface flow.

The material for treating the container to block the superfluid surfaceflow may include, for example, fluorine-containing material asexemplified by fluorine resins.

For example, very effectively used are a container made of teflon and acontainer coated on its surface with a fluorine resin. Also usable arethose comprising an organic polymer, graphite, glass or the like whosesurfaces have been subjected to surface treatment with CF₄ plasma or thelike.

The surface of a container may also be coated with a surface activeagent containing fluorine, or a wax comprised of pentadecafluorocaprylicacid, paraffin or the like.

Also effective for blocking the surface flow is a container comprisingpolyethylene or other organic polymers formed on its surface in layers,or a container made of polyethylene.

Using such materials, lining may be applied to the inner wall, or outerwall, of a container, or, alternatively, as illustrated in FIG. 1, theabove surface flow blocking material may be applied in a stripe to anupper part of the inner wall 12 above helium liquid surface 13, of acontainer Il.

Such materials are known as materials having water repellency, but ithas been hitherto unknown that these materials block the superfluidsurface flow.

On the other hand, examples of embodiments of the present invention inwhich uneveness for blocking the superfluid surface flow is provided mayinclude an embodiment in which, as illustrated in FIG. 2A, a streak likeprojection or groove 22 having a certain angle in the direction of theflow of the superfluid surface flow is provided on the inner wall orouter wall of a container 21.

FIG. 2B illustrates an embodiment in which a plurality of streak-likeprojections or grooves 25, 26, 27 and 28 are provided on the inner wallor outer wall of a container 24.

FIG. 3A is a view illustrating a cross-sectional shape of the projectionillustrated in FIG. 2. In the figure, R represents curvature radius ofthe end of a projection 33, and R should be not more than 50 times,preferably not more than 5 times, and more preferably not more than 3times the thickness of a surface flow 32 in order to block thesuperfluid surface flow. Since commonly the thickness of the surfaceflow 32 is considerably thin, R is in the range of 1 μm or less,preferably 1,000 angstroms or less. The projection may be 1,000angstroms or more in height so as to be particularly effective for thesurface flow blocking.

An angle θ of the projection 33 illustrated in FIG. 3A may preferably be30° or more.

FIG. 3B is a view illustrating a cross-sectional structure of a groovefor the surface flow blocking.

In FIG. 3B, what substantially contributes the blocking of a surfaceflow 35 are edge portions 37. The depth H of a groove 35 may preferablybe 1,000 angstroms or more; the width of the groove 35, 1,000 angstromsor more; and curvature radiuses R₁ and R₂, in accordance with theconditions shown in FIG. 3A.

More specifically, in FIG. 3B, R₁ and R₂ should be not more than 50times, preferably not more than 5 times, and more preferably not morethan 3 times the thickness of the surface flow 36. Since commonly thethickness of the surface flow 36 is considerably thin, R₁ and R₂ eachare desired to be 1 μm or less, preferably 1,000 angstroms or less.Angles θ₁ and θ₂ of the edge portions 37 each may preferably be 30° ormore.

The above uneveness for blocking the surface flow may be formed inplurality on the inner wall or outer wall of the container asillustrated in FIG. 2B, so that there can be exhibited further superioreffect.

Eventually, since such projection or groove can not necessarily beformed with ease, it is difficult to form a perfectly defect-freeprojection or groove. For this reason, the liquid helium climbs over theprojection or groove at a defective portion thereof, resulting in nosufficient achievement of the blocking of the superfluid surface flow.The blocking of the surface flow may be hindered also when foreignmatters such as dust are adhered on the projection or groove.

Such problems can be solved by providing a plurality of projectionsand/or grooves.

Also, if the quantity of liquid helium in a container decreases to lowerthe liquid surface level of the liquid helium, there is produced spacingbetween the liquid surface level and a projection or groove, and thewall surface corresponding to the spacing is covered with superfluidhelium. Then, the surface area of the liquid helium is enlarged toincrease the amount of evaporation of liquid helium, resulting inincrease in the consumption of liquid helium.

However, providing a plurality of projections and/or grooves in theinner wall from the part near to the mouth of the container to the partnear to the bottom thereof can bring the surface flow of liquid heliumto be blocked at a projection or groove near to the helium liquidsurface level, so that the increase in the surface area can beprevented.

As methods for forming the above uneveness (projection or groove), theremay be employed, for example, a cutting technique used when adiffraction grating is formed, or a method in which photolithography andetching are combined.

In the manufacture of IC, questioned is the undercut that may occur inetching. In the present invention, however, such undercut may rather beutilized when the projections are formed, so that there can be formedprojections being small in R.

The method of the present invention for storing volatile substance is byno means limited to the storing of a small amount of liquid helium in alaboratory. For example, a particularly great effect can be achievedwhen it is utilized upon cooling of large superconductive coils used innuclear fusion, electric power storage, etc. by liquid helium. Thereason is that, in such a large cooling system, bubbles are formed byvaporization of helium, and the bubbles are brought to stagnate at aparticular part of the cooling system, thereby suppressing the coolingat that part, or the formation of bubbles bring about destruction or thelike of the system owing to the increase of pressure. If superfluid isto be used in such a large scale apparatus, it is possible to carry outcooling with good efficiency by utilizing its high thermal conductivity.

If superfluid helium is utilized in analytical equipments, it is alsopossible to feed liquid helium to any desired place without using a pumpand to make it useful for cooling a specimen. In the case when an inertsurface is required to be formed in a vacuum reaction apparatus, it isfurther possible to form in a vacuum vessel of about 10⁻⁴ Torr the inertsurface covered with helium, if the liquid helium is previously cooleduntil its vapor pressure reaches, for example, 10⁻⁴ Torr or less. Suchmethods can be utilized in treatment of the inner surfaces of reservoirsfor hydrogen atom gas. In the case when a large quantity of liquidhelium is transported, the method of the present invention may beapplied to liquid helium tanks of tank trucks or tanker boats, so thatthe loss of liquid helium during transportation can be made very small.

Unnecessary loss of liquid helium can be made minimized in trafficfacilities utilizing a superconductivity phenomenon, such as linearmotor cars utilizing the magnetic float achieved by superconductivecoils.

Shown below is an example -n which the above described techniques forstoring superfluid helium is applied to the flow control of the surfaceflow of superfluid helium.

As illustrated in FIG. 4, for example, a superfluid helium transportingmember 45 comprising fluorine resin coated areas 41 and a fluorine resinnon-coated area 42 are dipped in a superfluid helium liquid 44. As aresult, a surface flow 43 flows wherein a limited region held betweenthe fluorine resin coated areas, i.e., the fluorine resin noncoated area42.

The projection as illustrated in FIG. 3A may also be used in the manner,for example, as illustrated in FIG. 5A, so that a surface flow 53 comingfrom a superfluid helium feeding portion 54 flows wherein a limitedregion (a superfluid helium transporting area 52) held between twostreak-like projections 51a and 51b.

EXAMPLE

The present invention will be described below in more detail withreference to Examples.

Example 1

As illustrated in FIG. 6, a block 62 made of foamed styrol was placed indouble Dewar vessels 63 and 64 for liquid helium, and a measuringcylinder 61 (300 cc) made of teflon was placed thereon.

Liquid nitrogen was introduced into a space 65 between the double Dewarvessels 63 and 64. The inner Dewar vessel 63 thereafter was sufficientlycooled, and then liquid helium was filled in the inner Dewar vessel 63,thereby cooling the inside of the inner Dewar vessel 63.

After the whole was sufficiently cooled, residual liquid helium in theinner Dewar vessel 63 was removed, and liquid helium was poured into themeasuring cylinder 61 made of teflon and placed in the inner Dewarvessel 63, which was hermetically closed with a cover. Subsequently, anexhaust vent 68 was connected to a rotary pump and the inside wasevacuated so that the inside of the Dewar vessel may come to be about 1Torr.

Liquid helium 66 was temporarily, vigorously boiled in the course of theevacuation, but became stable when the temperature of the liquid heliumwas lowered to 2.17°K. Evacuation to 1 Torr brought its temperature toabout 1.3°K. While maintaining its pressure to a constant level underthese conditions, how the liquid he11um decreased was examined. Theresult revealed that the helium decreased in the proportion of about 6cc/hr. Even after 15 hours, the liquid helium at 1.3°K decreased only by100 cc or so.

When a lead container was used in place of the container made of teflon,the liquid helium decreased at a rate of about 100 cc/hr, showing thatit decreased far more rapidly as compared with the case when thecontainer made of teflon was used.

Example 2

Using the same double Dewar vessels as in FIG. 6 except that the blockmade of foamed styrol and the measuring cylinder were not used andliquid nitrogen was directly poured into the Dewar vessel 63, the insidethereof was evacuated by means of a pump. At this time the temperatureof the liquid helium was 55°K and the liquid nitrogen had been a solid.

Even after the nitrogen was left to be stored under this condition forone month, the weight of solid nitrogen in the Dewar vessel 63 decreasedonly by 20%.

On the other hand, when it was stored at a temperature of approximately77°K without evacuation, the vessel become empty after 3 days.

Example 3

Example 1 was repeated to store liquid helium, except that the measuringcylinder made of teflon was not used and instead paraffin was applied toan upper part of the inner wall of the Dewar vessel, and also thepressure was adjusted to 0.1 Torr.

As a result, the liquid helium decreased by a rate of about 10 cc/hr. Onthe other hand, when the liquid helium was stored without coatingparaffin, it decreased at a rate of about 150 cc/hr.

Example 4

In Example 1, a measuring cylinder made of glass was used in place ofthat made of teflon.

Streak-like projections were formed by etching at intervals of 5 mm inthe peripheral direction of the inner wall surface of the measuringcylinder.

Next, the whole vessel was cooled With use of liquid helium, and liquidhelium was further poured into the measuring cylinder, followed bycooling under reduced pressure. Cooling to about 1.3°K brought thepressure to be lowered to 1 Torr or less. When the liquid helium wasstored under these conditions it decreased at a rate of 15 cc/hr.

For comparison, in the case when a measuring cylinder provided with noprojection was used, the liquid helium decreased at a rate of 80 cc/hror more.

Example 5

In the container 61 illustrated in FIG. 6, a superconductive coil of 20cm in diameter, comprised of Nb-Ti, was placed in the manner such thatits lead wire for feeding electric current may extend out of thecontainer.

The space 65 of the double Dewar vessels was filled with liquidnitrogen, and liquid helium was poured into the teflon containerreceiving the coil, followed by cooling under reduced pressure to lowerthe temperature of liquid helium to 1.2°K. The saturated vapor pressureat this time was 1 Torr or less.

Next, a persistent current was allowed to flow to the coil, but no heatgeneration from the coil was seen.

At this time, the liquid helium in the teflon container decreased at arate of 20 cc/hr. On the other hand, when such cooling under reducedpressure was not carried out, the helium decreased at a rate of 200cc/hr or more.

Example 6

A guide 55 for transportation, as illustrated in FIG. 5B, was used fortransporting superfluid helium. It was made of glass. The edges of theguide were provided with grooves 51a and 51b satisfying the requirementsof FIG. 3B (i.e., θ₁, θ₂ = 40°; L = 200 μm; H = 200 μm) which were cutin parallel by photolithography. The whole guide was sufficiently cooledto 2.17°K or less, and thereafter superfluid helium was fed to a liquidhelium transporting path 52 formed inside the guide 55. As a result, asurface flow was produced in the direction along the liquid heliumtransporting path 52 as shown by an arrow 53, whereby the liquid heliumcould be transported.

Example 7

As illustrated in FIG. 7A, using a tube of 1 mm in diameter, made ofpolytetrafluoroethylene, prepared was a tube 72 for transporting liquidhelium from a liquid helium container 71 in a cryogenic test apparatus71 to a smaller specimen-cooling system in the apparatus. The container71 has a volume of 1.5 liters and is made of polytetrafluoroethylene,and the tube 72 is securely fixed to the container 71. This tube 72 is30 cm long from the container 71 to the specimen cooling system, and thewhole apparatus is held in a large Dewar vessel. A twisted wire of about1 mm in diameter, obtained by bundling and twisting copper wires of 0.05mm in diameter, is inserted to the tube 72, and this twisted wire 73made of copper extends from the tube 72 and is immersed in superfluidhelium contained in the container 72.

In feeding liquid helium to a specimen 76 on a specimen stand 77 byusing the apparatus illustrated in FIG. 7A, the whole was first cooledto 2.17°K or less, and the twisted wire 73 made of copper wassufficiently immersed in superfluid helium 74. As a result, heliumreached the specimen 76 along the surface of the twisted wire 73. Thatis, the superfluid helium did not spread outside the tube 72 and flowedalong only the surface of the twisted wire 73. Namely, this apparatusmade it possible to stably feed the superfluid helium from the container71 to the specimen 76 without using any particular pump.

It was also possible to prevent the superfluid helium from rising upalong the inner side of the Polytetrafluoroethylene container 71 andfrom flowing outsIde from a container cap 75. It was confirmed thatsince helium ran along only the inside of the tube 72, it had a smallsurface area, and therefore its vaporization in the course oftransportation could be suppressed to a minimum. Also it was confirmedthat not only employment of copper in the form of a twisted wireenhanced transport velocity of the liquid helium but also adjustment ofthe number and diameter of the copper wires constituting the twistedwire enabled adjustment of the helium transport velocity.

Example 8

Using nickel, prepared was a superfluid helium container having the samepurpose as in Example 7 and used in a cryogenic test apparatus asillustrated in FIG. 7B, and also prepared using a pipe of 1/8 inch madeof nickel was a pipe 79 for transporting superfluid helium from thiscontainer 78 to a specimen-cooling system in the apparatus. A number ofstreak-like grooves 80 were cut on the inner wall of the container 78,substantially parallel to the liquid surface, to prevent the surfaceflow of helium from rising up and from overflowing outside the container78. A number of streak-like grooves 81 were also cut on the outside ofthe nickel pipe 79 to prevent helium from being transported along theoutside of the pipe 79.

According to the method mentioned above, the superfluid helium wastransported along only the inside of the nickel pipe 79 without anyparticular pump, and reached the specimen 76 with only minimumvaporization.

Example 9

As a specimen stand for imparting a temperature difference on specimensunder cryogenic temperature, prepared was one as illustrated in FIG. 8.The specimen stand 90 made of copper was supported by a specimen standsupport 86, whose end was immersed in superfluid helium 85. Specimens 87were arranged on the specimen stand 90. A heater 88 for controllingtemperature was fitted to an end portion of the specimen stand 90 sothat the stand may be electrically heated. The whole vessel was held ina thermal insulating container. The liquid helium 85 rose up from ahelium tank 84 along the specimen stand support 86, reached apolytetrafluoro-ethylene surface layer 82 provided to block thesuperfluid surface flow, and stopped there. Then the specimen standsupport was cooled at the part adjacent to the layer 82. On the otherhand, heating of the specimen stand support by a heater 88 brought heatto be conducted from the heater area toward the specimen stand support86 to make temperature gradient on the specimen stand 90, so that thetemperature difference could be made among the samples 87 arrangedthereon. This suppressed the movement of the superfluid helium to theside heated by the heater, thus making it easy to effect heating by theheater and also making it possible to lessen the evaporation of helium,so that consumption of liquid helium could be suppressed to a minimum.

As described above, it has become possible according to the presentinvention to minimize the loss of volatile substances owing tovaporization.

Also, in the case when the volatile substance ia superfluid helium, thepresent invention can block its surface flow, and therefore the heliumdecreased at a rate lowered to one-several tenth as compared with theconventional methods.

It further has become possible to control the advance direction oradvance velocity of the surface flow by bringing at least a part of thesurface of a structure, on which the surface flow of superfluid heliumis caused, into a state in which the surface flow passage is suppressed.

We claim:
 1. A method for storing liquid helium in a container,comprising the steps of:storing the liquid helium in the container;maintaining the liquid helium at a temperature such that it is in asuperfluid state; and suppressing the superfluid surface flow of theliquid helium by blocking means disposed on a wall surface of thecontainer.
 2. The method of claim 1, further comprising the step ofmaintaining the saturated vapor pressure of the liquid helium at 300Torr or less.
 3. The method of claim 1, further comprising the step ofmaintaining the saturated vapor pressure of the liquid helium at 100Torr or less.
 4. The method of claim 1, further comprising the step ofmaintaining the saturated vapor pressure of the liquid helium at 50 Torror less.
 5. A method of claim 1, wherein the saturated vapor pressure ofthe liquid helium is maintained between 0.1 to 1 Torr.
 6. A method ofclaim 1, wherein the liquid helium is maintained at 2.17K or less.
 7. Amethod of claim 1, further comprising the step of treating the wallsurface of the container with a material capable of suppressing thesuperfluid surface flow.
 8. A method of claim 7, further comprising thestep of making the treating material water repellant.
 9. A method ofclaim 7, wherein the treating material is a wax or a fluorine-containingsubstance.
 10. A method of claim 1, further comprising the step ofproviding the wall with a projection or groove to suppress thesuperfluid surface flow of the liquid helium.